This invention relates to a circuit for noise reduction and, more particularly, to a circuit which can be used as a level-compressing encoder for signals which are to be recorded, and which can be used as a level-expanding decoder for signals which are reproduced from a record medium.
Noise reduction circuits are used in signal transfer systems, such as in signal recorders and the like, in order to reduce noise and distortion which would be accentuated by that system, whereby the effect of the noise reduction circuit is to expand the apparent dynamic range of that system. In a typical noise reduction circuit, an encoder is provided for those signals which are to be recorded, and a complementary decoder is provided for those signals which are reproduced. The encoder generally includes a level-compression circuit and a high frequency pre-emphasis circuit, wherein higher frequency components of an information signal to be recorded are emphasized, the emphasis level being inversely related to the information signal level. The decoder generally includes a level-expansion circuit and a high frequency de-emphasis circuit to perform a complementary operation on the information signals which are reproduced.
In the Dolby.RTM. noise reduction system, a low-level input signal is amplified with a substantially constant gain until that input signal reaches a predetermined level. Thereafter, the amplification of that input signal is reduced until yet another, higher level is reached, whereupon amplification with substantially constant gain is carried out once again. In addition to such amplification of the input signal prior to recording, an emphasis circuit is used in order to pre-emphasize the higher frequency components of the input signal. This overall operation generally is referred to as signal compression. After the input signal has been suitably compressed, it is recorded. A complementary signal expansion process is carried out when the aforementioned signal is reproduced. That is, the pre-emphasized high frequency components are de-emphasized, and the de-emphasized signal is amplified with a gain less than unity. This gain is substantially constant over a predetermined range of relatively low signal levels, and when the reproduced signal exceeds a predetermined level, the gain is increased until a still higher level is reached.
The aforementioned Dolby noise reduction system is of relatively simple construction; and this system has been used extensively in home entertainment systems, such as magnetic tape recorders/reproducers and the like. However, although the Dolby system results in some improvement in the dynamic range of the recorder/reproducer, this improvement generally is limited to be on the order of about 10 dB; and this improvement is apparent primarily in the frequency region which exceeds 1 KHz. Furthermore, the aforementioned changes in the gain of the level-compression and level-expansion amplifiers are non-linear. Because of this non-linearity in the gains, level matching between the encoding and decoding processes often is difficult. Hence, some distortion may be apparent for those signals having intermediate signal levels.
Another noise reduction system is the so-called DBX.RTM. system. This system is described in U.S. Pat. No. 3,789,143. One advantage of the DBX system over the aforementioned Dolby system is that the gains of the amplifiers which carry out the signal compression and signal expansion operations, that is, the signal expansion and signal compression ratios, are substantially constant, regardless of the signal level of the input information signal. For example, prior to the recording operation, the input information signal is compressed with a constant compression ratio k. When the compressed signal subsequently is reproduced, the reproduced signal is expanded with a constant ratio 1/k, that is, at an expansion ratio which is the reciprocal of the compression ratio. Since constant compression and expansion ratios are used throughout the signal level range, that is, the non-linearity found in the Dolby system is avoided, level matching between recorded and reproduced signals is easily attained. Moreover, in the DBX system, the apparent improvement in the dynamic range of the recorder/reproducer is on the order of about 40 dB. Also, desirable noise reduction is achieved over substantially the entire audio frequency range of 20 Hz to 20 KHz.
However, the particular compression and expansion characteristics of the aforementioned noise reduction systems generally are obtained primarily for constant input signal levels, that is, signal levels which do not undergo abrupt transients. Stated otherwise, the advantages attained by these noise reduction systems are a function primarily of the static characteristics thereof. Difficulties are found in the dynamic transient characteristics of such systems. For example, if an information signal to be recorded exhibits a relatively low signal level, the gain, or compression ratio, of the encoder amplifier may be relatively high. Now, if this information signal undergoes an abrupt increase in its signal level, that is, it undergoes a large positive transient, the gain of the amplifier, or compression ratio, will not be reduced as rapidly as the signal level increases. Hence, although the gain, or compression ratio, should be reduced when processing the high-level information signal, in actuality it remains at its prior high level. Consequently, the strong transient is amplified with relatively large gain, thereby resulting in a compressed signal that exhibits "overshoot". That is, the level of the compressed signal is far too large. This high-level signal, when recorded, results in saturation of the magnetic medium, thereby causing distortion in the signal which is recorded and in the information which ultimately is reproduced therefrom.
Another disadvantage of the aforementioned noise reduction systems is that they may be subject to so-called noise modulation. In noise modulation, noise components are varied as a function of input signal level variations. Such changes in the noise components, or noise modulation, is highly perceptible and is quite distracting when it accompanies a reproduced audio signal. This phenomenon is pronounced when the frequency components of the input signal are noticeably different from the noise frequency component. For example, if the information signal is an audio signal representing the sound of a piano, noise modulation is heard separately and distinctly, and is not masked even if the volume level of the information signal is increased.
One proposal for reducing noise modulation in a noise reduction circuit is described in U.S. Pat. No. 4,162,462. In this proposal, the higher frequency components of the information signal are pre-emphasized prior to recording when the information signal exhibits low and medium signal levels, and relatively little pre-emphasis is provided when the information signal exhibits higher levels. When the information signal processed in the foregoing manner is reproduced, the higher frequency components are subjected to relatively high de-emphasis when the reproduced signal exhibits low and medium signal levels, and these higher frequency components are subjected to relatively low de-emphasis when the reproduced signal is at a higher level. Although this proposal reduces the undesired effects of noise modulation, saturation of the magnetic record medium due to overshoot in the compressed signal nevertheless is present.
In order to overcome the aforenoted disadvantage presented by overshoot, it also has been proposed to increase the speed of response of the level-compression circuitry. However, if the response speed is increased, an improvement in eliminating overshoot is accompanied by deterioration in the noise modulation characteristic. Another proposal in preventing overshoot is described in copending application Ser. No. 151,154, filed May 19, 1980. As will be apparent from the ensuing description, the present invention is an improvement over the noise reduction circuit described in this copending application.
Another proposal of a noise reduction circuit which minimizes overshoot contemplates the use of a plurality of substantially similar noise reduction circuits connected in parallel. Each noise reduction circuit is intended to operate over a selected portion of the frequency spectrum of the input information signal. The outputs of these individual noise reduction circuits are combined, or mixed, resulting in an overall level-compressed information signal suitable for recording. However, the use of a plurality of parallel-connected noise reduction circuits is relatively complex and expensive. For example, if n such noise reduction circuits are used, the overall cost of the noise reduction system is n times the cost of a noise reduction system in which only a single noise reduction circuit is used.